Lower the speed of I2C for DDC/CI

[Maciej Wójcik <w8jcik@xxxxxxxxx>]

Hello,

DDC/CI is an interface that allows changing of display settings like brightness, contrast, colors, selection of input.

DDC/CI implemented in i915 driver operates over I2C with a hard-coded speed of 100KHz.

This speed works for most of the displays, but not all of them.

Would you consider lowering the default speed of I2C to support additional displays?

Does the change below look acceptable, or do you expect some issues with it?

diff --git a/drivers/gpu/drm/i915/display/intel_gmbus.c b/drivers/gpu/drm/i915/display/intel_gmbus.c
index e9e4dcf34..88aba021e 100644
--- a/drivers/gpu/drm/i915/display/intel_gmbus.c
+++ b/drivers/gpu/drm/i915/display/intel_gmbus.c
@@ -200,7 +200,7 @@ bool intel_gmbus_is_valid_pin(struct drm_i915_private *i915, unsigned int pin)
 
 /* Intel GPIO access functions */
 
-#define I2C_RISEFALL_TIME 10
+#define I2C_RISEFALL_TIME 5
 
 static inline struct intel_gmbus *
 to_intel_gmbus(struct i2c_adapter *i2c)
@@ -916,7 +916,7 @@ int intel_gmbus_setup(struct drm_i915_private *i915)
 		bus->adapter.retries = 1;
 
 		/* By default use a conservative clock rate */
-		bus->reg0 = pin | GMBUS_RATE_100KHZ;
+		bus->reg0 = pin | GMBUS_RATE_50KHZ;
 
 		/* gmbus seems to be broken on i830 */
 		if (IS_I830(i915))

I have tested this change by recompiling my system kernel. The change fixes DDC/CI for one of my displays, while not breaking any other displays that I had.

What would be even more flexible is some way to change the speed run-time or boot-time. I can try to implement it if you suggest what mechanism to use for that. Otherwise, would you accept a simple change of lowering the speed?

Related issue https://gitlab.freedesktop.org/drm/i915/kernel/-/issues/11528

--
Maciej Wójcik